Heretofore, there have been known technologies of improving the hardness and the modulus of a rubber by reinforcing the rubber with staple fibers such as aramid and cellulose, or with a crystalline polymer such as syndiotactic polybutadiene, thereby increasing the complex modulus (E*) at for example 70° C. to improve steering stability (see, for example, Japanese Patent Laying-Open No. 2005-133025 (patent document 1)).
Japanese Patent Laying-Open No. 2005-133025 (patent document 1) proposes a rubber composition composed of a diene-based rubber component, starch and cellulose for the purpose of providing a rubber composition excellent in abrasion resistance. It also proposes to use bacterial cellulose as the cellulose. However, the technologies of patent document 1 are problematic in poor rupture characteristics and large energy loss at the interface between a rubber and cellulose because of poor compatibility of the rubber and the cellulose.
Japanese Patent Laying-Open No. 2005-075856 (patent document 2) discloses a rubber composition prepared by incorporating a fine powdery cellulose fiber prepared from natural vegetable fiber into a diene-based rubber as a rubber composition that can exert low repellency and rigidity (steering stability) simultaneously. However, the technology of patent document 2 has some room for improvement for obtaining rigidity and reinforcing property suitable for the incorporated amount of cellulose fibers for reasons of being short of fiber length of a cellulose fiber in a production method.
Therefore, it is still difficult to obtain a vulcanized rubber composition that is environmentally conscious and excellent in rupture characteristics and that causes less energy loss.    Patent document 1: Japanese Patent Laying-Open No. 2005-133025    Patent document 2: Japanese Patent Laying-Open No. 2005-075856